The invention concerns apparatus for the production of a web of fiber, e.g. a paper web, by dewatering a liquid suspension of the fibrous material between a first or inner rotating strip that is directed over a convexly curved supporting device and a second or outer rotating strip which is curved concavely over the suspension and which is held under tension longitudinally. The two strips rotate or move together through the dewatering area and carry the suspension between them. At least one of the strips is water permeable. This apparatus would typically be used in a paper making machine.
The permeable one of the two strips is usually like a sieve. When it is in the form of a metallic screen, the permeable strip is called a wire. The other strip may also be water permeable, e.g. it may be a wire or it may be a fabric screen, called a felt. Apparatus such as these when used in paper making machines are familiarly known as twin wire formers, crescent formers, etc.
These apparatus as opposed to the usual long wire apparatus, have the advantage that dewatering of the suspension can occur from both sides simultaneously and there is no free surface or unpressured portion of the suspension during the formation of the web. By providing an area for the formation of the web which has a constantly decreasing radius, it is sought to reduce the length of the dewatering area of the apparatus. (See German Published Specification Offenlegungsschrift 2,059,962). Also, it is sought to avoid accumulation of water and the consequent relatively limited flows at the intake or the entrance to the dewatering area as well as uncontrollable shearing flows in the area of drainage, at an angle to the strips. (Journal "Das Papier", 31. Year of Publication, Volume 10A (1977), Pages V 125 to V 137).
Through the above-described means, separation of the fibers as a flocculent during the formation of the web is almost totally eliminated. As a result, the web produced is much more uniform than those formed on prior long wire apparatus. However, in practice, the improved effect is often not achieved. The inventor has recognized that during the formation of the web between two simultaneously co-rotating strips (normally wires), undesirable relative longitudinal motions occur between the liquid suspension being carried between the strip and the layers of fibrous material which have already been deposited on the strips. These motions cause a cloudy appearance in and the formation of nodular shapes in the web produced.
For this reason, the twin wire web forming apparatus has only been acceptable for producing a very limited range of types of paper and webs and only for high speed operation, or for producing the kind of products which do not need a high level of uniformity, as for example for the inner layers of multi-ply board.